189 research outputs found
Transient jets in the symbiotic prototype Z Andromedae
We present development of the collimated bipolar jets from the symbiotic
prototype Z And that appeared and disappeared during its 2006 outburst. In 2006
July Z And reached its historical maximum at U ~ 8.0. During this period, rapid
photometric variations with Dm ~ 0.06 mag on the timescale of hours developed.
Simultaneously, high-velocity satellite components appeared on both sides of
the H-alpha and H-beta emission line profiles. They were launched
asymmetrically with the red/blue velocity ratio of 1.2 - 1.3. From about
mid-August they became symmetric. Their spectral properties indicated ejection
of bipolar jets collimated within an average opening angle of 6.1 degrees. We
estimated average outflow rate via jets to dM(jet)/dt ~
2xE10-6(R(jet)/1AU)**(1/2) M(Sun)/year, during their August - September
maximum, which corresponds to the emitting mass in jets, M(jet, emitting) ~
6xE-10(Rjet)/1AU)^{3/2} M(Sun). During their lifetime, the jets released the
total mass of M(jet, total) approx 7.4x1E-7 M(Sun). Evolution in the rapid
photometric variability and asymmetric ejection of jets around the optical
maximum can be explained by a disruption of the inner parts of the disk caused
by radiation-induced warping of the disk.Comment: 31 pages, 9 figures, 2 tables, accepted for Ap
Thermoelectric cross-plane properties on p- and n-Ge/SixGe1-x superlattices
Silicon and germanium materials have demonstrated an increasing attraction for energy harvesting, due to their sustainability and integrability with complementary metal oxide semiconductor and micro-electro-mechanical-system technology. The thermoelectric efficiencies for these materials, however, are very poor at room temperature and so it is necessary to engineer them in order to compete with telluride based materials, which have demonstrated at room temperature the highest performances in literature [1].
Micro-fabricated devices consisting of mesa structures with integrated heaters, thermometers and Ohmic contacts were used to extract the cross-plane values of the Seebeck coefficient and the thermal conductivity from p- and n-Ge/SixGe1-x superlattices. A second device consisting in a modified circular transfer line method structure was used to extract the electrical conductivity of the materials. A range of p-Ge/Si0.5Ge0.5 superlattices with different doping levels was investigated in detail to determine the role of the doping density in dictating the thermoelectric properties. A second set of n-Ge/Si0.3Ge0.7 superlattices was fabricated to study the impact that quantum well thickness might have on the two thermoelectric figures of merit, and also to demonstrate a further reduction of the thermal conductivity by scattering phonons at different wavelengths. This technique has demonstrated to lower the thermal conductivity by a 25% by adding different barrier thicknesses per period
Si/SiGe bound-to-continuum quantum cascade emitters
Si/SiGe bound-to-continuum quantum cascade emitters designed
by self-consistent 6-band k.p modeling and grown by low energy
plasma enhanced chemical vapour deposition are presented
demonstrating electroluminescence between 1.5 and 3 THz. The
electroluminescence is Stark shifted by an electric field and
demonstrates polarized emission consistent with the design.
Transmission electron microscopy and x-ray diffraction are also
presented to characterize the thick heterolayer structure
Ge/SiGe parabolic quantum wells
Quantum wells with parabolic confining potentials allow the realization of semiconductor heterostructures mimicking the physical properties of a quantum harmonic oscillator. Here we report the attempt of attaining such parabolic quantum wells (PQWs) within the Ge/SiGe material platform. Multiple PQWs featuring different widths and composition have been epitaxially grown and characterized by means of high-resolution x-ray diffraction and scanning transmission electron microscopy. The compositional profile is seen to deviate slightly from an ideal parabola, but the quantum confined states are almost equally spaced within the valence and conduction band as indicated by photoreflectance measurements and k . p modelling
Flat metamorphic InAlAs buffer layer on GaAs(111)A misoriented substrates by growth kinetics control
We have successfully grown, through the detailed control of the growth kinetics, flat InAlAs metamorphic buffer layers on 2 degrees -off GaAs(111)A substrates using molecular beam epitaxy. Almost full plastic relaxation is obtained for a layer thickness > 40 nm. The control of an adatom diffusion length and a step ejection probability from the bunches permits a reduction of the InAlAs epilayer root-mean-square surface roughness to 0.55 nm
Probing the in-plane electron spin polarization in Ge/Si0.15 Ge0.85 multiple quantum wells
We investigate spin transport in a set of Ge/Si0.15Ge0.85 multiple quantum wells (MQWs) as a function of the well thickness. We exploit optical orientation to photogenerate spin-polarized electrons in the discrete energy levels of the well conduction band at the Γ point of the Brillouin zone. After diffusion, we detect the optically oriented spins by means of the inverse spin-Hall effect (ISHE) taking place in a thin Pt layer grown on top of the heterostructure. The employed spin injection/detection scheme is sensitive to in-plane spin-polarized electrons, therefore, by detecting the ISHE signal as a function of the photon energy, we evaluate the spin polarization generated by optical transitions driven by the component of the light wave vector in the plane of the wells. In this way, we also gain insight into the electron spin-diffusion length in the MQWs. The sensitivity of the technique to in-plane spin-related properties is a powerful tool for the investigation of the in-plane component of the spin polarization in MQWs, which is otherwise commonly inaccessible
Erratum to: Photoluminescence Study of Low Thermal Budget III–V Nanostructures on Silicon by Droplet Epitaxy
Extending the emission wavelength of Ge nanopillars to 2.25 μm using silicon nitride stressors
The room temperature photoluminescence from Ge nanopillars
has been extended from 1.6 μm to above 2.25 μm wavelength through
the application of tensile stress from silicon nitride stressors deposited by
inductively-coupled-plasma plasma-enhanced chemical-vapour-deposition.
Photoluminescence measurements demonstrate biaxial equivalent tensile
strains of up to ~ 1.35% in square topped nanopillars with side lengths of
200 nm. Biaxial equivalent strains of 0.9% are observed in 300 nm square
top pillars, confirmed by confocal Raman spectroscopy. Finite element
modelling demonstrates that an all-around stressor layer is preferable to a
top only stressor, as it increases the hydrostatic component of the strain,
leading to an increased shift in the band-edge and improved uniformity over
top-surface only stressors layers
Photoluminescence Study of Low Thermal Budget III–V Nanostructures on Silicon by Droplet Epitaxy
We present of a detailed photoluminescence characterization of high efficiency GaAs/AlGaAs quantum nanostructures grown on silicon substrates. The whole process of formation of the GaAs/AlGaAs active layer was realized via droplet epitaxy and migration enhanced epitaxy maintaining the growth temperature ≤350°C, thus resulting in a low thermal budget procedure compatible with back-end integration of the fabricated materials on integrated circuits
Expanding the Ge emission wavelength to 2.25 μm with SixNy strain engineering
Photoluminescence up to 2.25 μm wavelength is demonstrated from Ge nanopillars strained by silicon nitride
stressor layers. Tensile biaxial equivalent strains of up to ~1.35% and ~0.9% are shown from 200 × 200 nm, and
300 × 300 nm square top Ge pillars respectively. Strain in the latter is confirmed by Raman spectroscopy, and
supported by finite element modelling, which gives an insight into the strain distribution and its effect on the
band structure, in pillar structures fully coated by silicon nitride stressor layers
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